1. Field of the Invention
The present invention relates to an optical system.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Currently, with stages that require a long range of travel in one degree of freedom de-centering effects due to the guiding and driving mechanisms are either ignored, included in the budget, or compensated by using additional actuators. In some cases, when the guide mechanism allows it, as is in the case of magnetic bearings, an external reference, such as an interferometer can be used to measure, and compensate for, intrinsic de-centering effects in the guide and drive mechanisms.
De-centering in two or more degrees of freedom can be caused by material deformation due to thermal effects, stress, or caused by manufacturing tolerances limited by current technology and price. Other sources of de-centering effects can be due to the nature of the drive and/or the guide mechanism technology. For example, when using roller bearings and leadscrews as the drive mechanisms, some motion in axes perpendicular to the direction of travel can cause de-centering. Still other causes of de-centering can be deformation of the guide mechanism due to manufacturing, stress, thermal effects, de-centering forces in the drive mechanism, and vibration,
One optical design for an optical maskless lithography (OML) illuminator uses multiple optical elements that have a travel range, in one axis, of up to about 700 mm with de-centering sensitivities below about ±200 nm in the directions perpendicular to the axis of travel. An interferometer, or similar device, can be used in these systems to measure the actual de-centering of the lens and compensate for the optical effects using an additional set of optics. However, these tolerances have to be maintained for days, if not weeks, in between calibrations. Also, given the sensitivity of the optical elements to temperature changes, it is desirable to keep most, if not all, heat generating components, such as motors, away from the optical path. In addition, some optical elements may need to maintain alignment with the optical path in the directions perpendicular to travel in the sub micron range.
Therefore, what is needed is a system and method that compensate for de-centering of optical elements in an optical system.